1. Field of the Invention
The invention relates to the art of color image rendering. It finds particular application where an image created on or prepared for rendering on a first or source device is rendered on a second or destination device.
2. Description of Related Art
When an image is prepared for rendering on an electronic device the image is represented as a set of pixels. Each pixel describes a small portion of the image in terms of colorant pixel values for the colorants available on the rendering device. For example, typically a cathode ray tube (CRT) based computer display screen is comprised of red (R), green (G) and blue (B) phosphors. An image prepared for display on a CRT is described with a set of pixels. Each pixel describes the intensity with which the red, green and blue phosphors are to be illuminated on a small portion of the CRT. A similar procedure is followed when an image is prepared for rendering on a printing device. Currently, at least some color printing devices apply cyan (C), magenta (M), yellow (Y), and sometimes black (K) colorants to a print medium, such as paper or velum, in order to render an image. Such printing devices are said to operate in a CMY or CMYK color space. When an image is prepared for rendering on a color-printing device, the image is represented as a set of pixels. Each pixel describes a small portion of the image by calling for an appropriate mixture of the available colorants. Typically, the pixel value for each colorant can range from 0 to 255. The higher a colorant""s pixel value is, the more of that colorant the color image processor applies to the print medium. In a system employing 8-bit precision for the colorant signals, the number 255 represents the maximum or fully saturated amount of colorant. The number 0 is used when none of a particular colorant is required. It should be noted that sometimes, for the purposes of analysis or discussion this range is normalized to a range of 0 to 1.
In a CRT operating in RGB (red, green, blue) space, fully saturated red is described by pixel calling for R=255, G=0, B=0. In a printer operating in CMYK (cyan, magenta, yellow, black) space, fully saturated red is described by a pixel calling for C=0, M=255, Y=255, K=0. Magenta and yellow colorants combine through simple subtractive mixing and are perceived as red. There is no guarantee that the red described in RGB space and displayed on the CRT is the same red described in CMYK space and printed on a page. In fact, it is quite likely that the spectral properties of the red phosphor used in the CRT will be different than the spectral properties of the subtractively mixed magenta and yellow colorants of a particular printer.
As mentioned above, the CRT and the CMYK printer use different materials to generate the perception of color. The materials used impact a set of colors that each device can reproduce.
The set of colors a device can produce is referred to as the color gamut of the device. There is no guarantee that a color that can be produced by a first device can also be produced by second device. This is even true when both devices are CMYK printers.
Where color matching is required between two devices such as the CRT operating in RGB space and the printer operating in CMYK space, transforms based on careful calibration and measurement are required. In such a situation it is possible, for example, that the pure red RGB CRT pixel mentioned above, is mapped to a CMYK printer pixel calling for a less than fully saturated magenta component and a small amount of a cyan component. For example, the CMYK version of the original RGB red pixel referred to above might call for C=27, M=247, Y=255, K=0. Furthermore, if one wants to print a copy of the original pure red RGB CRT pixel on a second printer it is quite likely that a second transform will have to be used. That transform may translate the original RGB CRT pixel to a second CMYK pixel. For example, the second transform may map the original RGB CRT pixel to a second CMYK pixel calling for C=20, M=234, Y=240, K=35. One reason two different CMYK printers may require different transforms is that different printers use different colorants. For example, a first magenta colorant used in a first printer may have a different spectral content than a second magenta colorant used in a second printer. Likewise, a first yellow colorant used in a first printer may have a different spectral content than a second yellow colorant used in a second printer.
From the foregoing discussion it can be seen that an image prepared for rendering on a first device may need to be transformed if it is to be properly rendered on a second device. Such a transformation is an attempt to emulate the first or source device onto the second or destination device. In order to achieve spectral content matching, the emulation of the color gamut of the CRT on the first CMYK printer caused the red CRT pixel to be mapped to a first CMYK pixel calling for C=27, M=247, Y=255, K=0. The emulation of the color gamut of the CRT on the second CMYK printer caused the red CRT pixel to be mapped to the second CMYK pixel calling for C=20, M=234, Y=240, K=35. Obviously, therefore, even where there is no RGB CRT image involved, an image prepared for printing on the first printer may have to be transformed before its spectral content can be matched on the second printer. In such a situation the first printer is said to be emulated on the second printer.
For example, when, a photographic image has been prepared for rendering on a first CMYK device, for example a Standard Web Offset Printing (SWOP) device, but must then be rendered on a second CMYK device, for example, a xerographic printer, a xe2x80x9c4 to 4xe2x80x9d transform is typically used to emulate the first device on the second device.
In order to generate the 4 to 4 transform, a color characterization profile is needed for both devices. Each color characterization profile maps a calorimetric space, such as CIELAB to the device""s color gamut. The mapping is bi-directional, so that each device""s color gamut can also be mapped to the calorimetric space. The source image, the image prepared for printing on the first device, is transformed from the first device""s CMYK space, via the first device""s color characterization profile, into calorimetric space e.g. CIELAB. The calorimetric version of the image is then transformed via the second device""s color characterization profile, into the second device""s CMYK space.
Spectral matching, however, is not always the desired goal when rendering color images. For example, when rendering business graphics, such as pie charts and bar charts, a user is concerned with how vivid and pure the colors in the chart are and not with how well the rendered colors match a set of original colors.
Business graphics are most often composed of primary colors. For the purposes of this discussion the primary colors include red, green, blue, cyan, magenta, yellow, black and white. Red, green and blue are considered primary colors because they can be additively mixed to produce the perception of other colors in the human eye. Cyan, magenta, and yellow are considered primary colors because the human eye also perceives their subtractive mixture as other colors. White is perceived when red, green and blue are mixed in a well-balanced manner. Likewise, black is perceived when cyan, magenta and yellow are mixed in a well-balanced manner. Additive mixing of any two of red, blue and green produces one of cyan, magenta and yellow. Subtractive mixing of any two of cyan, magenta and yellow produces one of red, blue and green. For example, as indicated above, a balanced mixture of magenta and yellow is perceived as red.
In business graphics applications, the exact shade of color, for example, red, produced is not an issue. What is required is that the red produced appears pure and even. This is easily achieved when only one or two colorants are used to produce a color. When a third or fourth colorant are added, for example, in an attempt to match spectral content, the color can be perceived as uneven, dull and impure.
Furthermore, in a system employing colorants cyan (C), magenta (M), yellow (Y), and black (K) colorants, the rendition of dark vivid colors in business graphics applications is often best achieved by using not more than two of the colorants C, M, Y to produce the desired vividness, along with K to produce the desired darkness. That is, every pixel is rendered in 3-colorant combinations of CMK, MYK, or CYK. In such cases, contamination with a fourth colorant is undesirable, as it could reduce the vividness or purity of the color.
It is best therefore, when rendering business graphics, to accept the idiosyncrasies of the rendering device in exchange for a clean vivid appearance. The user is usually not concerned with how well the red on the rendered chart matches the red on the computer screen or the red as it was printed the week before on a different printer. The user usually just wants a pure red. If a transform is used, that attempts to emulate the source or original device on a new rendering device, the results can appear muddy or dirty. Therefore, currently, when one wants pure hues, such as when rendering business graphics, it is often better not to use a correcting transform. Instead one accepts the transform that is the inherent characteristic of the rendering device and makes a selection during a system configuration step that turns off the use of correcting transforms. The inherent characteristics of a particular rendering device are called the device transform.
When absolute or relative spectral accuracy between portions of an image are the dominating factor, then of course the user can make a selection to use correcting or emulating transforms. Requiring the user to make processing technique selections is problematic. In some instances the user does not have the expertise required to make informed processing technique decision. Where the user has the required expertise the process is still tedious and time-consuming. Furthermore, some images do not fall neatly into the category that clearly requires emulation transformation or the category for which only the device transform should be used. Some images contain both components that are best left untransformed and components that are best rendered through the use of emulation transformation.
Therefore a processing method is needed that consists of different techniques to accommodate the needs of different images and smoothly moves between techniques as the needs of a given image require.
To that end, a new method and a device for rendering an image have been developed. The method takes an image comprised of pixels that has been prepared for rendering on a first device and prepares it for rendering on a second device. The method can be applied where the first device has an associated first color gamut, and where the first color gamut can be subdivided into at least a first sub-gamut, a second sub-gamut and a transition region between the first sub-gamut and the second sub-gamut. The second device also has an associated second color gamut that can be subdivided into at least a first sub-gamut, a second sub-gamut and a transition region. The method comprises the steps of finding each pixel""s location within the first color gamut, mapping pixels located in the first sub-gamut of the first color gamut under a first rendering intent through a first transform to pixels within the first sub-gamut of the second color gamut, mapping pixels located in the second sub-gamut of the first color gamut under a second rendering intent through a second transform to pixels in the second sub-gamut of the second color gamut, and mapping pixels located between the first sub-gamut and the second sub-gamut of the first color gamut via a blend of the first transform and the second transform.
One advantage of the present invention is that it allows images to be rendered properly without user intervention.
Another advantage of the present invention is that it can be used to process images that contain both pictorial and business graphics type components.
Another advantage of the present invention is that it renders images in a more pleasing manner than do prior art techniques.
Another advantage of the present invention is that it preserves neutral colorant components of an image. This is especially useful in business graphics.
Another advantage of present invention is that it preserves primary color components of an image. This is also especially useful in business graphics.
Another advantage of present invention is that it preserves the purity and vividness of dark vivid colors in a business graphics image.